The present invention relates to a fuel assembly, a reactor core, and a method for loading of the fuel assembly, especially, relates to the preferable fuel assembly being loaded in boiling water type nuclear reactors (hereinafter called BWR) for increment of reactor shut down margin, improvement of fuel economy, and maintenance of reactivity control, and relates to the preferable reactor core loaded with the fuel assemblies and the preferable method for loading of the fuel assembly.
A conventional fuel assembly which is used in a BWR is generally composed of a plurality of fuel rods and one or a plurality of water rods which are arranged in a channel box by being supported at an upper end and a lower end of the fuel rod and the water rod with an upper tie plate and a lower tie plate.
During operation of the reactor, slightly unsaturated cooling light water enters from a hole of the lower tie plate into an interval among the fuel rods in the fuel assembly, and flows out from a hole of the upper tie plate as vapor-liquid two phase flow after being heated by the fuel rods and boiled during flow from lower portion to upper portion of the fuel rod interval. As the result, void fraction of the coolant increases from 0% at the lower portion to about 70% at the upper portion of the fuel assembly. Consequently, the ratio of hydrogen atoms to heavy metal atoms; that is, the ratio of moderator to fuel (H/U ratio), which is an important factor for determining of nuclear characteristics of the fuel assembly alters remarkably depending on a position in an axial direction.
On the other hand, it is necessary to install control rods and instrument tubes for neutron detectors exterior of the channel box in the BWR and, therefore, such intervals (hereinafter called water gap) as to enable the above mentioned rods and tubes be inserted are provided between the fuel assemblies. The water gap is filled with saturated water and, consequently, effects of the saturated water existing in the water gap to the fuel rods in the fuel assembly are different depending on whether the fuel rods locate at periphery of the fuel assembly (a region near the water gap) or central region of the fuel assembly. That is, the peripheral region of the fuel assembly near the water gap has larger H/U ratio than the central region. Accordingly, such H/U ratio as an important factor for determining the nuclear characteristics of the fuel assembly differs depending on the radial position in the fuel assembly.
The H/U ratio is a parameter to determine an average energy of a neutron. As the ratio becomes larger, the average energy of the neutron becomes smaller (softer neutron spectrum), and the nuclear fission reaction with nuclear fissile material is enhanced. Concurrently, the softening of neutron spectrum increases the neutron absorbing reaction by the moderator (light water as coolant) as well as the nuclear fission reaction. Accordingly, there is an optimum H/U ratio in view of fuel economy. Moreover, fuel rod power generation which depends on the reactivity of nuclear fission is determined by the H/U ratio. That is, in view of thermal margin and controlability of excess reactivity of the fuel assembly, it is necessary to consider the H/U ratio.
On the other hand, with related to conventional nuclear reactors, extension of an operation cycle of the reactor and high burn up of fuel are considered for increasing of a plant utilization factor and effective utilization of uranium resources. For increasing of discharged burn up of the fuel assembly, it is necessary to increase enrichment of the fuel assembly. The increment of the fuel enrichment influences the optimum H/U ratio. Further, the extension of loading period of the fuel assembly in the reactor means that the fuel is effected under different H/U ratios for a long period in the reactor, and the above mentioned influence of the H/U ratio is enhanced.
In regard to improvement of distribution of the H/U ratio in a radial direction and an axial direction of the fuel assembly, there are such methods as enlarging of a saturated water region at a necessary portion and regulation of distribution of the nuclear fissile material. The former is a method for improving the H/U ratio by enlarging the saturated water region at the central and the upper region of the fuel assembly, wherein moderating effect of the neutron is deteriorated. And the latter is a method for improving the H/U ratio in the axial direction by regulation of loading quantity of the fuel.
For example, in JP-A-62-211584 (1987), a method to increase horizontal cross sectional area at the upper region in the axial direction of the fuel assembly and to arrange a water rod having a horizontal cross section of cruciform at the upper region in the axial direction is proposed. Short length fuel rods are loaded beneath the cruciform protruded region of the cruciform water rod.
And, in JP-A-52-50498 (1977), a method to arrange fuel rods having different length in order to form a flow channel of coolant having reversely tapered shape toward the upper region in the axial direction of the center of the fuel assembly is disclosed.
In USP-4,968,479, a fuel assembly for achieving high burn up by increasing of fuel enrichment is disclosed. The fuel assembly is composed of a water rod having larger horizontal cross sectional area at the upper region in the axial direction than the area at the lower region and of fuel rods having three kinds of different length in order to reduce an increment of local power peaking accompanying with using of the highly enriched fuel with a burnable poison at the beginning of operation and to optimize a reactivity distribution at the upper and the lower region of the fuel assembly during a designated operation period. The shortest fuel rod is arranged at the position adjacent to the lower small diameter region of the water rod, and contains fuel having equal to or lower enrichment than the fuel assembly average enrichment, the medium length fuel rod contains fuel having equal to the fuel assembly average enrichment, and a part of the longest fuel rods contain fuel having the burnable poison (column 15, line 25-60, FIG. 22, 30B-30D).
Further, in JP-A-63-311195 (1988), on a fuel assembly for achieving high burn up by increment of fuel enrichment, an improving method for increasing the reactor shut down margin in considering that the increasing of the enrichment at the upper region of the fuel assembly increases the reactivity of the upper region at the reactor shut down margin is disclosed. The fuel assembly improved by the above described method has two water rods each of which have a large diameter and uniform horizontal cross section in the axial direction and fuel rods, which are arranged adjacent to the two large diameter water rods, containing lower enriched fuel at least at the upper region of the fuel rod than the fuel in other next fuel rods.
Other prior techniques relating to the increment of burn up are disclosed in USP-4,229,258, JP-A-63-21589 (1988), and JP-A-64-28587 (1989). In USP-4,229,258, a fuel assembly having higher enriched fuel at the upper region than at the lower region is disclosed. In JP-A-63-21589 (1988), a fuel assembly in which high enriched fuel rods are arranged at the outermost periphery in the horizontal cross section and the enrichment at the lower region in the axial direction of the fuel rods is higher than the enrichment at the upper region is disclosed. In JP-A-6428587 (1989), a fuel assembly in which enrichment of fuel pellets in fuel rods containing enriched uranium and gadolinium is the highest in the fuel assembly and the effective fuel length of the fuel rod is shorter than the length of fuel rods containing enriched uranium but not gadolinium is described.
Further, in JP-A-53-43193 (1978), a conventional method in which the saturated water region at the upper region of the fuel assembly is increased by making the thickness of the channel box wall thin at the upper region of the fuel assembly is disclosed.
Among above described prior techniques, the conventional method disclosed in JP-A-63-311195 (1988), wherein large water rods having uniform horizontal cross section in the axial direction are used, improves the distribution of the moderator to fuel ratio (H/U ratio) at the upper region of the fuel assembly. Nevertheless, the improvement of the distribution of the H/U ratio at the lower region of the fuel assembly is not considered in the conventional method.
In accordance with the prior art wherein the improvement of the H/U ratio distribution in the axial direction of the fuel assembly is aimed at, the characteristics at the lower region of the fuel assembly is sacrificed for the improvement of the H/U ratio distribution in the axial direction and, consequently, the improvement of the H/U ratio distribution in the radial direction at the lower region of the fuel assembly is not sufficient. And the distribution of the moderator and fuel materials (fissile materials and parent materials) in the axial and the radial direction is not considered sufficiently in the prior art.
That is, in the methods disclosed in JP-A-62-211584 (1987), JP-A-52-50498 (1977), and USP-4,968,479, the horizontal cross sectional area of water rod or moderator flow channel at the upper region in the axial direction of the fuel assembly is made larger than the area at the lower region in order to increase the H/U ratio at the upper region of the fuel assembly. But the methods have such problems that the cross sectional area of the water rod at the lower region is not sufficient, and flattening of thermal neutron flux distribution is not achieved sufficiently. The problems cause lowering of the fuel economy.
Moreover, in the methods disclosed in JP-A-62-211584 (1987) and JP-A-52-50498 (1977), when the enrichment of the short fuel rods arranged in a region which is yielded by decreasing of H/U ratio at the lower region of the fuel assembly is excessively high, fissile materials are generated more at the lower region than at the upper region of the fuel assembly and, consequently, a large peak in power distribution is caused at the lower region of the fuel assembly. Accordingly, there are such problems that stability becomes insufficient and fuel economy is lowered by increasing of average void fraction in the axial direction of the fuel assembly.